Medicaments are often administered to a patient by means of a syringe in which the user depresses the exposed end of a plunger journalled axially in the syringe body to drive a piston journalled within the bore of the syringe body a given axial distance to eject a quantity of fluid from the syringe body through a needle mounted at the distal end of the syringe body, ie. that end removed from the end at which the user acts upon the exposed end of the plunger shaft. The amount of fluid ejected will depend upon the extent of movement of the plunger and this is typically assessed by the user monitoring a number of clicks or other audible signals generated as the plunger is moved. Alternatively, the user can monitor the extent movement against a fixed scale or the like.
Where a single dose is to be administered, the user often draws sufficient fluid into the barrel of the syringe from a bottle or the like by retracting the plunger to draw fluid into the body of the syringe through the needle and ejects the whole of that dose by a single depression of the plunger. However, this is inconvenient where repeated doses of the same medicament are to be administered. It is therefore customary for a syringe to incorporate a cartridge or other reservoir so that a user can administer a number of dosages without the need to replace the source of the medicament after each dosage. The syringe mechanism is therefore designed to permit repeated actuation of the plunger and means must be provided for enabling the plunger to be moved the desired amount for each dose and, usually, for the user to select the dosage to be administered. However, this carries the risk that the user may administer an over or under dose by incorrect selection of the amount of medicament discharged from the cartridge or reservoir, and/or by repeated partial actuation of the device, for example where the user has a tremulous hand.
Various forms of dosage selection mechanism have been proposed, see for example European Patent Application No 0037696 A1 and British Patent Application No 2109690 A. However, such mechanisms rely upon the accurate selection of the dosage by the user and accurate operation of the device if consistent dosages are to be achieved.
It has been proposed in, for example British Specification No 1230522, to form a number of tangentially and axially off set radial projections on the shaft of the plunger, the axial off set of each of which corresponds to the axial movement of the plunger required to administer a selected dose. The proximal end of the syringe body carries a transverse plate through which the plunger shaft passes. The plunger can be moved axially until the distal end of one of the projections engages the face of the transverse plate and thus halts further axial movement of the plunger. The plunger must then be rotated about its longitudinal axis to bring the projection into alignment with a radial slot in the plate which allows the projection to pass axially through the plate and thus allow further movement of the plunger until the next projection engages the face of the plate. Such a device provides means for discharging single doses from the syringe, However, the user can administer partial doses by incomplete depression of the plunger and can retract the plunger during use, for example by operating the plunger tremulously, to administer an unknown dose of medicament.
It has been proposed in European Patent Application No 0225439 A1 to provide the plunger of a syringe with an axial series of radially projecting teeth on each side thereof and to pass the plunger shaft through a transversely moveable block member which carries a second series of teeth adapted to engage with the teeth on the plunger. The block member is urged radially by a spring so that the teeth on the block engage those on the plunger and thus prevent axial movement of the plunger. When the block member is depressed against the spring, the engagement between the teeth is broken to allow the plunger to move axially. In order to limit the forward movement of the plunger, there is provided a spring loaded arm pivoting about a transverse axis, the arm carrying a third set of axial teeth which partially engages with the teeth on the top face of the plunger. By virtue of the arc of swing of the arm, the teeth it carries progressively engage with the teeth on the plunger as the plunger moves axially forward, until the teeth are fully engaged and prevent further axial travel of the plunger.
Such a mechanism is said to restrict forward movement of the plunger to a series of pre-set incremental movements. However, the mechanism requires concerted movement of the block and arm members to alternately secure and release the plunger. Furthermore, the plunger can be withdrawn an indeterminate distance when the teeth of the block member are disengaged from the teeth on the plunger. A user therefore does not know how much potential forward movement of the plunger is available in the device and hence the dosage which will be administered.
We have devised a form of syringe operative mechanism which reduces the above problems.